Here is the MS site that discuses additional licenses for XP. 
http://www.microsoft.com/windowsxp/home/howtobuy/addlic.mspx

My understanding is that the additional licenses can not be transferred 
if you sell or give a computer away. In essence, the licenses stay with 
you and do not go with the computers. The few dollars you save on each 
licenses could end up not being worth it. Getting a separate legitimate 
OS for each computer may be the better way to go.

Tom Mayer
 

Mick Fitzpatrick wrote:
> Hello
>
> Can someone please tell me what is the easiest and most cost effective way
> to legally put XP home on 4 PC's? I already have it running on one machine
> do I just install it on the others?
>
> I believe one can buy a multi-licence that is a lot cheaper than buying XP 4
> times?
>
> Any advice will be appreciated
>
> TIA ... Mick
>
>         The NOSPIN Group has added a new feature on our website,
>            web based bulletinboard for questions and answers:
>               Visit our sister website at http://nospin.com
>
>   

        The NOSPIN Group has added a new feature on our website,
           web based bulletinboard for questions and answers:
              Visit our sister website at http://nospin.com